Abstract
Induction of apoptosis by paclitaxel was investigated in human leukemic U937 cells. Treatment of U937 cells with 20 nM paclitaxel for 24 h induced apoptotic cells in 35-45% of cells, but were rescued by pretreatment of caspase inhibitor (Z-VAD-fmk) and KN-62, a CaMKII specific inhibitor. It was found that caspase-1 and caspase-3 were involved in the apoptotic pathway. Synchronous cells at different cell cycle stages exhibited differential involvement of caspases toward paclitaxel, and the reversion by caspase inhibitors was also varied with stages. Both caspase-1 and -3 were activated at the stage of G1 phase, also their inhibitors partial rescued cells. However, the caspase-3 inhibitor rescued higher percentage of apoptotic cells in the G2/M-phase, which exhibited little response to general caspase inhibitor and caspase-1 inhibitor. In S phase, cells were only slightly recovered using these three caspase inhibitors. It suggests that paclitaxel induced apoptosis is mediated by caspase-1 and -3 and the mechanism is modulated differently from caspases through cell cycle. Furthermore, KN-62 can not only rescue apoptotic cells, but also reduce both caspase-1 and -3 activity in the cell assay. It is concluded that CaMKII is probably upstream in the signaling pathway that regulates caspase-1 and -3 in the paclitaxel-induced apoptotic pathway. And paclitaxel triggers death signals through different mechanisms which are cell cycle-specific.